This project determines the structure of neuronal and glial cytoplasm, particularly as it pertains to axoplasmic transport, and the organization of cytoplasm. Microtubules isolated from the axoplasm of the squid giant axon continue to support movements of organelles for many hours. However, actin filaments in other cells support similar movements of organelles. A protein translocator responsible for these organelle movements has been characterized, a 600 KD protein with 110 KD and 60-65 KD doublet peptides. This protein induces beads to move along purified microtubules in the presence of ATP. When a monoclonal antibody column (directed towards the 110 Kd subunit) is used to purify this translocator it has the same peptide components and supports similar movements. Based on its size, pharmacological properties, and asymmetrical shape seen by molecular shadowing, this translocator protein belongs to a new class of motility protein, which we call kinesin. Kinesin appears to be of general significance in cellular motility. The organelle movements induced by kinesin are always directed towards the plus ends of microtubules, a direction corresponding to anterograde axonal transport. We have now purified from the axoplasm of squid giant axons a high molecular weight protein which transports exclusively in the direction that would correspond to retrograde transport, back towards the cell body. We have also shown that this protein is a two-headed species of dynein. Kinesin powered movements track along single protofilaments of microtubules whereas dynein frequently moves from one protofilament to another.